DESCRIPTION (provided by investigator): In 90% of cases, healthy individuals infected with Mycobacterium tuberculosis (Mtb) remain asymptomatic. This implies that the host generates and maintains a partially protective immune response capable of containing bacterial replication. When this equilibrium is altered, TB disease can occur. Besides the involvement of innate immunity, an efficient TB immune response also depends on CD4+ T cells. HIV is one of the major risk factors for TB, altering TB immune responses allowing progression to active TB. The most obvious immune defect induced by HIV infection is a massive depletion of CD4+ T cells. However, the risk of TB is increased soon after HIV infection, before profound CD4 loss occurs. This suggests that HIV may also induce qualitative changes in CD4+ T cell function, weakening protective TB immune responses. However, the precise nature of mycobacteria-specific CD4+ T cells that confer immune protection remains elusive. It is therefore of crucial importance to define the type and balance of CD4+ T helper subsets that participate in the establishment of Mtb latency, and establish to what extent HIV disturbs this equilibrium even in immuno- competent individuals. In ongoing experiments, we have found that mycobacteria-specific CD4+ T cells from individuals with latent TB infection (LTBI) exhibit a diverse balance of T helper subsets, characterized by the co-expression of lineage-defining transcription factors. Importantly, we have found that these single-cell transcription co- expression patterns in mycobacteria-specific CD4+ T cells are dramatically perturbed by HIV infection. Given these findings, we hypothesize that in healthy individuals, mycobacteria-specific CD4+ T responses are tailored towards a particular balance of CD4+ T helper cell subsets required for ongoing TB containment, but that HIV infection derails the establishment or maintenance of this CD4+ T cell differentiation balance. To test this hypothesis, using both global transcriptional and flow cytometry-based approaches, we propose the following specific aims: 1) To define the spectrum of T helper differentiation in mycobacteria- specific CD4+ T cells and test whether this pattern differs in individuals with latent and active tuberculosis, and 2) to test how HIV infection alters the transcription factor co-expression pattern, lineage commitment and transcriptional program of mycobacteria-specific CD4+ T cells. We anticipate that this study will: 1) reveal the complexity and flexibility of mycobacteria-specific CD4+ T cell lineage commitment and 2) identify novel mechanisms by which HIV infection alters these immune responses. This project will lead to a better understanding of immune mechanisms of protection against TB and thus be influential in immune intervention against TB.